The recent trends toward lighter, smaller, and higher-density electronic products have increased the demand for various printing boards. In particular, the demand for flexible laminates (also referred to as “flexible printing circuit boards (FPCs)”) has shown a notable increase. A flexible laminate is constituted from an insulating film and a circuit formed from a metal foil disposed on the film.
Typically, a flexible laminate is produced by bonding a metal foil onto a surface of a substrate with an adhesive material of various kinds under heating and pressure, the substrate being a flexible film made from an insulating material of various kinds. Polyimide films and the like are preferred as the insulating flexible film, and thermosetting adhesives such as epoxy and acrylic adhesives are typically used as the adhesive material. Hereinafter, FPCs made using thermosetting adhesives are also referred to as “three-layer FPCs”.
Thermosetting adhesives are advantageous in that bonding at relatively low temperatures is possible. However, requirements for properties, such as heat resistance, flexibility, and electrical reliability, are becoming more stringent, and it is possible that three-layer FPCs using thermosetting adhesives will have difficulty in meeting these stringent requirements. In order to overcome this problem, FPCs (hereinafter also referred to as “two-layer FPCs”) using thermoplastic polyimide as the bonding layer or made by directly forming a metal layer on the insulating film have been proposed. The two-layer FPCs have properties superior to those of the three-layer FPCs, and the demand for the two-layer FPCs is expected to grow in the future.
The two-layer FPCs that have been suggested are classified into three types, (1) PVD two-layer, (2) cast two-layer, (3) laminate two-layer, which have various advantages and disadvantages respectively. Among theses three types of two-layer FPCs, the laminate two-layer FPC gains prominence over the others because of its high productivity. The laminate two-layer FPC is produced by laminating an electrically conductive metal foil on an adhesive sheet on one or each side of which an polyimide-based adhesive agent layer is formed. As an example of the PVD two-layer FPC, Patent Citation 2 discloses a flexible copper-clad circuit board in which a thermoplastic polyimide layer and a certain metal layer, namely, a copper layer, are formed on a thermosetting polyimide film. The metal layer is formed directly thereon by sputtering or plating. These methods, however, cannot provide sufficient adhesiveness between the thermoplastic polyimide layer and the metal layer.
Moreover, the two-layer FPC is generally required to have (i) a high soldering heat resistance to cope with lead-free soldering, and (ii) a dimensional stability to cope with high density packaging. The laminating step for the laminate two-layer FPC should utilize high temperature and high pressure because the adhesive is made of polyimide having high melting point. The high temperature and the high pressure deteriorate the dimensional change ratio after etching and after heat treatment. Various attempts have been made to attain better dimensional stability. For Example, Patent Citation 1 discloses a method in which a surface of a polyimide film is mechanically roughed before the polyimide film is laminated with an adhesive and a copper. Moreover, Patent Citation 3 discloses a copper-clad laminate that is developed to cope with high temperature at lamination. The copper-clad laminate of Patent Citation 3 uses a multi-layer polyimide film integrally comprising a substrate polyimide layer that is low in heat expansion, and a thin-layer polyimide on both sides of the substrate polyimide layer, each thin-layer polyimide having particular structure and Tg. The art disclosed in Patent Citation 3 is characterized in its step of providing an adhesive agent layer, and in the adhesive agent layer itself. Thus, the art disclosed in Patent Citation 3 does not provide a fundamental improvement in dimensional stability and its dimensional stability remained insufficient.    Patent Citation 1: Japanese Unexamined Patent Application Publication, Tokukaihei, No. 6-232553    Patent Citation 2: Japanese Unexamined Patent Application Publication, Tokukai, No. 2002-280684    Patent Citation 3: Japanese Unexamined Patent Application Publication, Tokukai, No. 2003-71982